For radio communication, e.g., in wireless radio communication systems such as Long Term Evolution (LTE) and New Radio (NR) according to the Third Generation Partnership Project (3GPP), a reference signal (RS) is transmitted to determine a state of a radio channel between a base station and a radio device or to track impairments induced by a local oscillator of a transceiver of the radio device. The definition of the RS depends on its use case. Several types of RSs are defined in a wireless radio communication system, each serving one or more purposes. For example, a RS defined and used for coherent demodulation of a physical layer channel is referred to as demodulation reference signal (DM-RS), a RS defined and used for acquiring channel state information (CSI) in a downlink (DL) from the base station to the radio device is referred to as CSI-RS and a RS defined for tracking of time and frequency differences between transmitter and receiver is referred to as tracking RS (TRS).
Each transmitted RS is based on a sequence of values that define the modulation symbols respectively transmitted on radio resource elements (REs) configured or allocated for the RS. The sequence needs to be carefully selected to enable low cross-correlations between different RSs, i.e., RSs based on different sequences. For example, interference between RSs transmitted from different transmission and reception points (TRPs) should be mutually randomized. This means that a receiver should experience an interfering RS, i.e., a RS transmitted from another TRP not serving the receiver, as noise. This is typically achieved by assigning different sequences to different TRPs.
For the 3GPP NR waveform using orthogonal frequency-division multiplexing (OFDM) with a cyclic prefix (CP), it has been decided that the sequences defining the RSs shall be based on pseudo-random binary sequences (PRBSs) derived from Gold sequences. The desirable noise-like behavior is then achieved by assigning different PRBS seeds to different radio links.
However, the time dependency of the PRBS seed is such that the statistical behavior of the cross-correlations between RSs transmitted from a given pair of TRPs over time is not sufficiently randomized. Rather, the statistical behavior of the cross-correlations between RSs transmitted from a given pair of TRPs depends on the scrambling identifiers assigned to these TRPs. For example, only 213 sequences with consistent cross-correlation properties exist for 3GPP NR, although the scrambling identifier comprises more than 13 bits. As a consequence, a wireless radio communication system, such as 3GPP NR with 1008 different physical cell identifiers (PCIs), has only on the order of 8 unique sequences per cell.